A naphthalenedicarboxylic acid can be prepared by, for example, oxidizing a dialkylnaphthalene in the presence of cobalt, manganese and bromine. The crude naphthalenedicarboxylic acid obtained by this process, however, contains impurities, e.g., trimellitic acid and aldehydes, as well as cobalt, manganese and bromine derived from the catalyst. When such a naphthalenedicarboxylic acid is used as a starting material to prepare polyethylene naphthalate, coloring of the resulting polyethylene naphthalate or staining of a mold in the molding process may take place. Therefore, the naphthalenedicarboxylic acid obtained above needs to be purified prior to use.
As a method for purifying a naphthalenedicarboxylic acid, JP-A-1-110650 discloses a process for preparing purified bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate, which process comprises reacting an impure 2,6-naphthalenedicarboxylic acid with at least 2 mol of ethylene glycol based on 1 mol of the 2,6-naphthalenedicarboxylic acid in the presence of catalytic quantities of a tertiary amine and a titanium-containing compound to prepare bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate, crystallizing the bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate, and recovering the purified bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate.
JP-A-5-508870 discloses a process for preparing a purified dimethyl naphthalenedicarboxylate, which process comprises reacting 2,6-naphthalenedicarboxylic acid with methanol in an appropriate reaction region to prepare a reaction mixture containing dissolved dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate, cooling the reaction mixture to a temperature of not higher than about 40.degree. C. to crystallize main parts of the dissolved dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate, partitionating the crystallized dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate from the reaction mixture mother liquor, heating the partitionated dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate in a recrystallization solvent to a temperature high enough to dissolve at least a part of the dimethyl 2,6-naphthalenedicarboxylate and substantially all of the monomethyl 2,6-naphthalenedicarboxylate, recrystallizing the dimethyl 2,6-naphthalenedicarboxylate dissoloved in the recrystallization solvent at such a temperature that the dimethyl 2,6-naphthalenedicarboxylate is recrystallized while the main parts of the monomethyl 2,6-naphthalenedicarboxylate are retained in the recrystallization mother liquor, and partitionating the recrystallized dimethyl-2,6-naphthalenedicarboxylate from the recrystallization mother liquor.
Further, JP-A-7-173100 discloses a process for preparing a high-purity 2,6-naphthalenedicarboxylic acid, which process comprises dissolving crude crystals of 2,6-naphthalenedicarboxylic acid containing impurities in water in supercritical or subcritical state, cooling the resulting solution at a temperature of not higher than 300.degree. C. to precipitate crystals, and separating the crystals from the mother liquor at a temperature of 100.degree. to 300.degree. C.
In the circumstances, it is desired to develop a method for purifying a naphthalenedicarboxylic acid, wherein impurity contents can be further reduced by easier procedures.
The present invention has been made under such circumstances as described above, and it is an object of the invention to provide a method for purifying a crude naphthalenedicarboxylic acid, which method enables to obtain a mixture of a naphthalenedicarboxylic acid and a naphthalenedicarboxylic ester with reduced impurity contents, or a high-purity naphthalenedicarboxylic acid.